Yokogawa Centum PID parameters explained

Which direction makes the action stronger

On your system a LOWER proportional number means MORE aggressive proportional action. Proportional band is the inverse of gain.

Reset is in seconds/repeat, so a LOWER number means FASTER integral action.

This is the single most consequential thing to get right. Proportional band and gain move in opposite directions, and so do repeats-per-time and time-per-repeat. An engineer who has moved from one system to another and carried the old habit across will make loops worse while believing they are improving them.

• Proportional term: proportional band
• Reset units: seconds/repeat
• Rate units: seconds
• Algorithm form: series interacting

Series form and derivative interaction

In the series form the derivative term multiplies the proportional-plus-integral term rather than adding to it. The practical effect is that changing rate also changes the effective gain and reset, so the three parameters are not independent.

There is also a constraint that the ideal form does not have: a series controller cannot produce every combination the ideal form can. An ideal tuning set with an integral time less than four times the derivative time has no series equivalent at all. That is a real limitation of the structure rather than a rounding problem, and any honest conversion tool will tell you rather than returning a number.

Why the algorithm form matters

The algorithm form matters because the same three numbers behave differently in each. In the series (interacting) form the derivative term multiplies the proportional-plus-integral term, so changing rate also changes the effective gain and reset. In the ideal (non-interacting) form the three terms are independent of each other but all scaled by gain. In the parallel (independent) form even that scaling is absent — gain does not affect integral or derivative action at all. Moving a tuning set between systems without converting for form is one of the commonest ways a loop that worked on one plant fails on another.

Before you change anything

Confirm the units on the actual controller rather than trusting the platform default. Systems get configured in unusual ways, engineering units get changed during a migration, and a controller imported from another site may carry conventions from there.

The fastest check is to make a small deliberate change and watch which way the loop responds. If it gets more aggressive when you expected less, the convention is not what you assumed.

Questions that come up

What proportional band corresponds to a gain of one?

100%. The relationship is a reciprocal: band as a percentage equals one hundred divided by gain, so a gain of two is a band of 50% and a gain of one half is a band of 200%.

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